The present invention relates to a radio packet data communication device suitable in use for radio local area networks (LANs) and to a radio data communication method.
In most conventional communication systems, the modulation mode and the transmission rate are not changed from beginning to end during packet data communications via radio. However, in some communication systems, the modulation mode and the transmission rate are changed, if necessary, during communications. Such systems have a slow rate transmission mode (hereinafter referred to as a low rate mode) and a fast rate transmission mode (hereinafter referred to as a high rate mode). Communications are done using both packets each which is first transmitted in a low rate mode and then transmitted or received in a high rate mode and packets each which is transmitted or received in a low rate mode from beginning to end.
As an example of conventional data communications including a low rate mode and a high rate mode, there is radio data communication in which various flag signals and control signals are certainly transmitted in the low rate mode, and then communications are established using packets for transmitting information data in the high rate mode and packets for transmitting only an ACK (ACKnowledgement) signal in the low rate mode. FIG. 4 is a diagram showing the configuration of a packet for a mode selection in the conventional radio data communications. In the packet, the leading portion in a low rate mode is a preamble portion (training period) while the leading portion in a high rate mode is a preamble portion (training period). In the preamble portion, information data portion is demodulated after the carrier detection and synchronous detection.
The conventional radio data communication system always begins to communicate in the low rate mode. If the rate mode type is not notified in advance to the receiving side, the system cannot ensure communications only in the high rate mode. When communication for a large volume of information data such as VOD (video-on-demand) is desired after establishment of communication protocols, the operation of changing the low rate mode to the high rate mode must be repeated for each packet. This results in the system operation with poor efficiency.
As an another example, JA-P-150440/1992 discloses a radio data transmission system in which the transmission rate is selected. In this radio data transmission system used for the MCA (Multi-Channel Access) business activity radio or the like, a link establishment signal is simultaneously subjected to a MSK (Minimum Shift Keying) demodulation and an AM-MSK (AM-Minimum Shift Keying) demodulation. Thereafter, the data transmission mode upon transmission with the data format for MSK modulation or AM-MSK modulation is selected based on the detection condition of the frame synchronous signal of the link establishment signal in each of the MSK demodulation and the AM-MSK demodulation. Thus the current transmission rate is selected by transmitting the link acknowledgment signal with the selected format.
In the AM-MSK modulation corresponding to a high rate mode, demodulation is performed 2 bits by 2 bits. That is, one bit is used for the AM-demodulation while one bit is used for the MSK demodulation. Hence, since the bit rate is twice that of the MSK modulation corresponding to the low rate mode, the transmission efficiency is poor. Moreover, when the modulation mode is changed, the link must be established every time. Hence, this means that the data communication device in which a low rate mode and a high rate mode are selected in packet units has a poor transmission efficiency.